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Astron. Astrophys. 356, L13-L16 (2000)

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4. The cause of the `jumps' in polarization angle

Two processes can cause jumps in polarization angle [FORMULA] across the `canals': a sudden change in RM across the `canals', and a jump in intrinsic [FORMULA] of the emission incident on the Faraday screen. A large change in intrinsic [FORMULA] implies a change in magnetic field direction and is therefore quite difficult to understand in view of the absence of structure in total intensity I at the more than 2% level (see Sect. 2). On the other hand, variations in the RM of the Faraday screen would seem to be quite natural, if not unavoidable.

Discontinuities in RM must play an important rôle in producing the `canals', because the `canals', although similar in adjacent frequency bands, generally do not occur in all bands, and certainly are not identical in the different bands (see Fig. 2). This indicates that the jumps in [FORMULA] are mainly due to changes in RM. However, the question is if the jumps in [FORMULA] are indeed accompanied by jumps in RM of the right magnitude so that [FORMULA] is produced at the frequency where the `canal' is best visible.

In principle, the determination of RM only involves a simple linear fit of the polarization angles in the five frequency bands (at 341, 349, 355, 360 and 375 MHz) vs. [FORMULA], but in practice several complications may arise. First, the observed values of [FORMULA] may be biased due to imaging effects (like off-sets) in the Stokes Q- and U-maps from which [FORMULA] is derived (cf. Wieringa et al. 1993). Our data indicate that, in the maps of this region of sky, such off-sets are quite small, so that the bias in the [FORMULA] values is small. Second, it is not obvious that the assumption of pure Faraday rotation ([FORMULA]) is supported by the data (see Haverkorn et al. 2000).

In Fig. 4 we show an array of plots of [FORMULA] vs. [FORMULA] for independent beams in the small region (indicated in Fig. 2) that contains two clear `canals'. As can be seen, a direct determination of [FORMULA]RM across the `canals' is not at all trivial. Without knowing the position of the `canals', one probably would have some trouble to find the `canals' from discontinuities in RM distribution alone, due to the uncertainties in the RM-estimates, which sometimes are considerable. On the other hand, if one knows where the canals are one can identify some related `jumps' in RM.

[FIGURE] Fig. 4. Polarized intensity P at 349 MHz in the area indicated by the box in Fig. 2. Overlaid are small plots of [FORMULA] vs. [FORMULA] for independent points, with a linear fit through the data. RM's range from [FORMULA] -7 rad m-2 in the upper left corner to -1.5 rad m-2 in the `island' between the two `canals' (below centre).

From the present data, it seems quite likely that the `canals' are primarily due to quite abrupt and relatively large changes of RM, with [FORMULA]RM/RM ranging from [FORMULA] 0.3 to more than 1 (at least in this region of sky). Note that in this region the RMs are in the range from -10 to +10 rad m-2 (also confirmed by several polarized extragalactic radio sources in these same observations). However, a more robust conclusion about the relation between [FORMULA] and [FORMULA]RM requires a detailed analysis of more, and more sensitive data, and a careful error analysis.

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© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000